Method and apparatus for mixing additives into a fuel

ABSTRACT

Embodiments of the present invention relate to a method and apparatus for mixing additives into a fluid fuel at a predictable concentration. The method comprises: taking a sample of the fuel; mixing the additive into the sample in metered proportions; testing the sample to determine that the correct amount of additive is present; storing the remaining fuel until it is time for the fuel to be used; and mixing the additive into the remainder of the fuel in the same metered proportions.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for mixing additivesinto a fuel at a predictable concentration.

BACKGROUND TO THE INVENTION

In a bid to reduce the carbon output of motor vehicles, many governmentsare promoting the use of biofuels as a replacement for petrol. A biofuelwill not necessarily produce less carbon than petrol when it is burnt,but since such fuels are typically derived from newly-grown plants oranimal products, and since those plants and animals will have absorbedatmospheric carbon directly or indirectly throughout their life, the netcarbon output of the fuel can be greatly reduced.

In the US and European markets, suppliers are already required to add aproportion of bioethanol to all grades of gasoline/petrol. In the UK forexample, under the Renewable Transport Fuels Obligation, suppliers offossil fuels must ensure that a specified percentage of the road fuelsthey supply is made up of renewable fuels. This percentage is currently3.6929%.

The level of ethanol to be dosed into any particular gasoline blend isdriven by marketing and environmental issues. Frequently the supplierwill want to control the ethanol levels in gasoline very precisely inorder to ensure that they do not add too little, and so break the law,or too much, and so waste resources.

This task is made more challenging by the fact that ethanol is volatile,like most fuels, and its effects on gasoline may be difficult topredict. This is especially problematic if the fuel is to be stored fora long period of time. Therefore merely mixing 5% ethanol with 95%gasoline does not guarantee that the resulting biofuel will eventuallypass a test as containing 5% ethanol.

Therefore a system for reliably mixing ethanol and gasoline would bevery useful.

SUMMARY OF THE INVENTION

Viewed from one aspect, the present invention provides a method forproviding a fuel meeting a predetermined specification of properties.The method comprises taking a sample of the fuel; mixing an additiveinto the sample in metered proportions; testing the sample to determinethat the combination of the fuel and the additive meets thepredetermined specification of properties; and storing the remainingfuel without the additive for subsequent mixing with the additive intothe remainder of the fuel in the same metered proportion.

In this way, the invention provides a method by which fuel can beadulterated reliably and in a repeatable fashion. Since the fuel is onlymixed with the additives when it is time for the fuel to be used, thereis little time for the characteristics of the fuel and the additives tochange after mixing. Nevertheless, the properties of the fuel incombination with the additive can be guaranteed as a result of themixing and testing carried out on a sample of the fuel. Since manyfuels, such as gasoline and ethanol, are inherently unstable, forexample hygroscopic, and may change over time due to, for example,evaporation and reactions with environmental pollutants, mixing at thelast possible moment ensures that the characteristics of the fuel remainhighly predictable.

One purpose of the method and apparatus described herein is to dosegasoline with a known volume of denatured ethanol prior to analysis ofthat gasoline by various analysers in a gasoline blending analyserhouse. The user can then predict the effect that adding ethanol togasoline at the depot will have on the various measured properties. Theapplicant has found that accuracies of ±0.02% can be achieved, enough tomeet legal requirements in the blending of ethanol with petrol. Thepredetermined specification of properties may include requirements forproperties such as vapour pressure, octane rating and the like, whichare commonly determined in order to certify a gasoline blend.

Viewed from an alternative aspect, the invention provides a method formixing additives into a fuel at a predictable concentration. The methodcomprises: taking a sample of the fuel; mixing the additive into thesample in metered proportions; testing the sample to determine that thecorrect amount of additive is present; storing the remaining fuel untilit is time for the fuel to be used; and mixing the additive into theremainder of the fuel in the same metered proportions.

Where necessary, the method may further comprise the step of adjustingthe metered proportions of the fuel and the additive where testingreveals that the sample contains an incorrect amount of additive, beforemixing the additive into the remainder of the fuel in the adjustedmetered proportions.

Typically, the fuel and the additive are fluids at the time of mixing.It is advantageous to use fluids where possible, as they are more easilymixed than solid fuels. However solid fuels can still be mixed, forexample when they are in the form of powders. One or both of the fueland the additive may be a liquid at the time of mixing.

Also typically, one of the fuel and the additive comprises ahydrocarbon, which may be distilled from crude oil and may be gasoline.Crude oil fractions, and in particular gasoline, are widely used andfrequently need to be mixed with precise quantities of other substancesbefore they are used. Therefore this method may be used for such fuels.

Typically one of the fuel and additives comprises alcohol, and thealcohol will typically comprise ethanol. So for example, the fuel may begasoline while the additive is ethanol, added to decrease the net carbonoutput of the fuel. However, it may also be the case that the fuel isethanol and the additive is petrol, where ethanol is for example to bedenatured in order to prevent human consumption. Other chemicals mayalso be present if required.

It may be that one of the fuel and the additive comprises a bio-fuel,such as a hydrocarbon derived from biomass, for example ethanol derivedfrom fermenting sugar derived from plants, or diesel derived fromvegetable oils and animal fats.

The invention also provides an apparatus adapted for carrying out themethods described above, the apparatus comprising: a blending system forincorporating the additive into the fuel; and a sample line for takingoff a sample of the mixed additive and fuel for testing.

The blending system may comprise a skid, which can allow the blendingsystem to be easily moved and installed.

Where the fuel and the additive are liquid at the time of mixing, theblending system will typically comprise a plurality of cylinders. Eachcylinder will contain a piston, and each cylinder will comprise at leastone inlet, through which fuel or additive is supplied to the cylinder.

Typically, each cylinder will comprise a first inlet and a second inlet,one at each end of the cylinder, and a valve which in use alternatelydirects the fuel or additive to the first inlet or the second inlet.With this arrangement, the piston of each cylinder may be driven solelyby the pressure of fluid entering the cylinder.

Also typically, the blending system will comprise a primary cylinder andat least one secondary cylinder, wherein the pistons in the secondarycylinders are arranged to operate in synchrony with the piston in theprimary cylinder.

Where the blending system comprises a primary and at least one secondarycylinder, and where each cylinder comprises a first inlet, a secondinlet and a valve as described above, the primary cylinder may compriseat least one proximity switch, arranged to operate the valve in theprimary cylinder when the piston approaches the end of the primarycylinder. Where this is the case, the proximity switch in the primarycylinder may also be arranged to operate the valves in the secondarycylinders.

Typically, the apparatus comprises a heat exchanger, and the fuel andthe additive are put through opposite sides of the heat exchanger priorto mixing. This helps to ensure that the fuel and the additive are thesame temperature when they are mixed, and allows them to be mixed moreaccurately.

Particularly with the volatile fluids used as fuels, changes intemperature can lead to changes in volume and other characteristics thatcan make controlling the proportions mixed difficult. Therefore thetemperature of a blending system according to the invention is typicallycontrolled, in order to control the temperature of the fuel and theadditive. The blending system, and in particular the cylinders whereused, may therefore be mounted inside a temperature controlledenclosure.

Advantages of these embodiments are set out hereafter, and furtherdetails and features of each of these embodiments are defined in theaccompanying dependent claims and elsewhere in the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the teachings of the present invention, andarrangements embodying those teachings, will hereafter be described byway of illustrative example with reference to the accompanying drawings,in which:

FIG. 1 is a diagram of an ethanol blending system according to theinvention; and

FIG. 2 is an illustration of an ethanol injection skid, also accordingto the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will now be describedwith particular reference to a method for incorporating additives into afluid fuel.

FIG. 1 shows an ethanol blending system 1 according to the invention.The ethanol blending system 1 is designed to produce a dosing accuracyof ±0.02% volume, so for example if the required dosing level is 5%ethanol by volume, the dosage level provided by the ethanol blendingsystem 1 will be in the range 4.98-5.02%.

The ethanol blending system 1 comprises two main parts, the ethanolstorage skid and the ethanol injection skid 2 (shown in more detail inFIG. 2). The ethanol blending system 1 is comprised of skids, i.e.constructed on pallets, so that it can be easily moved and installed.Once the blending system is connected to the required tanks of fuel itcan begin to operate without extensive installation or construction.

Generally the ethanol injection skid will be installed close to aprocess analyser so that fuel mixes can be quickly analysed to confirmthat they meet the required specification of properties. The ethanolstorage skid may be located with the injection skid or at some distanceaway, connected by pipes.

The ethanol storage skid comprises an ethanol storage tank 3. Thecapacity of the tank is determined by the needs of the user, andtherefore by the required ethanol dosage level for the relevant market,the demand of the installed analysers and the length of the gasolineblend run. Generally the tank capacity will be between 1,000 and 15,000litres. The ethanol storage tank 3 shown in FIG. 1 is made fromaustenitic stainless steel, but materials can be used depending on theuser's requirements.

The ethanol storage tank 3 is equipped with a nitrogen blanketing system4, which operates using dry plant nitrogen to maintain an overpressureof 5 millibars in the headspace above the ethanol. This is to preventwater being absorbed into the ethanol from the atmosphere surroundingthe tank.

Ethanol storage tanks according to the invention are typically fittedwith a visual level indicator and level monitoring systems so that theremaining ethanol can be easily determined. The ethanol storage tank 3,for example, is provided with a Guided Wave Radar (GWR) level detectionsystem 5 operating over the main length of the tank. This allows thelevel in the tank to be determined to within ±1 mm.

It is common for a denaturing agent to be added to the ethanol to renderit unfit for human consumption. Often, the denaturing agent is gasoline,and the concentration of the denaturing agent must be accuratelycontrolled as this accuracy will have an effect upon the accuracy of thefinal ethanol/gasoline blend. Therefore ethanol storage tank 3 isprovided with a temperature compensated mechanical positive displacementflow meter arrangement 6 with a pre-settable totaliser and automaticshut off valve to allow the user to add the required amount ofdenaturing agent. The accuracy of this flow meter is typically 0.05%.

Depending upon the capacity of an ethanol storage tank 3 according tothe invention it may be equipped with one or two single- or three-phaseelectrically driven pump units 7, 8 complete with contactors and thermaloverload trips. These pumps will be used either singly or in conjunctionto mix the tank contents and provide a pressurised supply of ethanol tothe injection skid.

All electrical components in the ethanol blending system 1 are suppliedwith suitable hazardous area certification to meet the client's localrequirements, which will vary from country to country.

The ethanol blending system 1 also comprises an ethanol injection skid2, which will mix gasoline from the user's process line with therequired amount of (often denatured) ethanol and supply the mixture atthe required pressure and flow rate to meet the requirements of theinstalled on-line gasoline analysers.

The ethanol injection skid 2 is shown in FIG. 2, and comprises a set ofprecision volumetric cylinders and pistons 11, 12, 13, 14, 15 with aminimum certified accuracy of ±0.02%. The master gasoline cylinder 11represents a volume of 100%. This cylinder reciprocates and works as adouble acting pump. The motive force to operate the master gasolinecylinder 11 is provided by the gasoline pressure in the process sampleline, so no other energy source is required. High pressure samples enteron one side of the cylinder and force samples out of the other side to alower pressure, so supplying the fuel to the rest of the ethanolinjection skid 2 and eventually to the analyser house. The mastergasoline cylinder 11 comprises solenoid valves 16 suitable for switchingthe high and low pressure sides of the cylinder and proximity switchesattached to the piston shaft. Therefore, at the end of each stroke ofthe piston, the proximity switches will activate solenoid valves 16,redirecting the pressure so that the piston is driven back along thecylinder. In this way the master gasoline cylinder 11 can operateautomatically and continuously without any outside power source.

The master gasoline cylinder 11 is attached to two secondary cylinders12, 13. In other embodiments of the invention, there may be only onesecondary cylinder, or there may be three or more. Each secondarycylinder has a volume corresponding to the fraction of the volume of thegasoline master cylinder that gives the ethanol percentage required. Inthe ethanol injection skid 2 the user can choose between a mix of 4.6%ethanol and a mix of 7.6% ethanol by selecting which secondary cylinderto use. The user can also select a mix of 12.2% by using both cylinders.

In use, the secondary cylinders 12, 13 reciprocate with the mastergasoline cylinder 11 and use solenoid valves 16 activated by the sameproximity switches to act as double acting pumps. In a multi cylinderinstallation such as the ethanol injection skid 2, any cylinder which isnot in use will just re-circulate denatured ethanol back into theethanol supply line from the ethanol storage tank 3. The cylinder whichis in use will pump a metered supply of denatured ethanol into a staticmixer 17, along with the metered gasoline, for supply to the analyserhouse.

The user is therefore able to select which secondary cylinder is used,and hence the volume of ethanol added. The user can also bypass thesystem altogether if non-ethanol-containing gasoline is required.

Piston stroke length errors are eliminated in the ethanol injection skid2, as all cylinders are rigidly coupled and are controlled by the sameproximity switches. Therefore any variation in stroke length will applyequally to all cylinders, and so mixing ratios will be maintained.

To maintain the required accuracy it is important that during thepumping cycles the gasoline and ethanol are at the same temperature toavoid errors due to their differential thermal expansion rates.Therefore, to minimise temperature differentials the ethanol andgasoline flow through opposite sides of a heat exchanger 18 and thecylinders are mounted inside a temperature controlled enclosure 19. Thetemperature of the ethanol and gasoline are monitored by the controlsystem 20 of the ethanol blending system 1, and the system is designedto maintain the temperatures of the two fuels closer than 2° C.

To eliminate any potential errors due to differences in thecompressibility of gasoline and ethanol the gasoline and ethanolpressure is also monitored by the control system 20, and the ethanolpressure can be electronically controlled to track the gasoline pressureat a level of 0.5 bars or better.

There are two ways to check the accuracy of the master gasoline cylinder11 and the secondary cylinders 12, 13. Firstly, a pair of checkcylinders 14, 15 and pistons are provided. The output of the mastergasoline cylinder 11 can be used to fill the larger of the checkcylinders, the gasoline check cylinder 15. To ensure that the pistons ofthe gasoline check cylinder are at the zero position, and to prevent anyvaporisation, the opposite side of the pistons is slightly pressurisedwith nitrogen or air, in use. When the gasoline check cylinder is fullthe extension of the piston shaft, which is proportional to the volumein the check cylinder, is measured by an LVDT (Linear VariableDifferential Transformer) sensor 21 and compared to a set point value.Any deviation beyond the set point value will be notified by the controlsystem.

The second smaller check cylinder is the ethanol check cylinder 14, andis used for checking the ethanol delivery. It operates in exactly thesame way as the gasoline check cylinder 15, except that it is providedwith one LVDT sensor 21 for each secondary cylinder 12, 13. Each LVDTsensor 21 measures a set point corresponding to the ethanol dosing levelof a particular secondary cylinder 12, 13.

This procedure of checking the cylinders can be carried out manually, orthe control unit 20 can initiate periodic tests automatically accordingto user preference.

Another way to test the accuracy of the ethanol injection skid 2 is todye the ethanol while it is still in the ethanol storage tank 3, using asuitable dye such as Sudan Blue, which is a common petroleum dye. Areference blend of gasoline and dyed ethanol is then made at the desiredaccuracy level, and this blend is used as the reference for a highlysensitive dual channel online spectrophotometer tuned to monitor thelight absorbance of the dye. The measuring channel of the spectrometermonitors the output from the ethanol injection skid 2. Any difference inlight absorbance will be directly proportional to the difference inethanol concentration. This enables a continuous check to be carried outof the blending accuracy. Alternatively, this procedure can be used as aspot check using a laboratory-based spectrophotometer.

Early seal leak detection is carried out by a flammable vapour detectorinstalled within the injection skid housing close to the cylinders.

The ethanol blending system 1 also comprises a control unit 20. Thecontrol unit 20 is based on a rugged fan-less industrial PC with solidstate memory, eliminating the need for more vulnerable magnetic harddrives. The PC operates under the Linux operating system. The GUI(Graphical User Interface) is provided by an armoured 17″ glass touchscreen 22, suitable for direct operation by gloved or un-gloved hands.The control unit 20 accepts analogue and digital inputs from all partsof the system including the ethanol storage tank 3, and provides theoperator with an overview of the system status on a graphic mimicdisplay. The operator can control the system fully from the GUI, andcarry out tasks such as selecting the gasoline grades and setting alarmlevels. The control unit 20 maintains a record of the number of cylinderand valve operations as an aid to maintenance. The control unit 20 doesnot control the cylinder operation, as this is done by hardwiredproximity switches and relays, but the unit does monitor all theassociated parameters. The control unit 20 is mounted in an explosionproof box suitable for the local hazardous area rating. The touch screen22 forms part of the explosion proof box.

While an ethanol blending system 1 according to the invention can beprovided with some hardwired alarms if required, for example for a highor low tank, or a fatal alarm, it is envisaged that the primary outputwill be via a RS485 modbus link 23 from the control unit 20. If requiredthe modbus link 23 may be used to control the system remotely, selectingthe grades and performing calibration checking on demand. At all timeslocal control will take precedence over remote control.

The control unit 20 will typically be located with the ethanol injectionskid 2. The ethanol storage tank is therefore provided with a localmimic display in order to display all parameters associated with thetank.

Accuracy is essential to the operation of the invention. Thereforeseveral measures have been taken to ensure that sufficient accuracy ismaintained. Most of these measures have been discussed above, but weoffer a more detailed discussion here.

Inaccuracy Due to Thermal Expansion in the Ethanol Storage Tank

The ethanol storage tank 3 will also expand and contract withtemperature and so will the ethanol contents with one effect counteringthe other. The tank expansion is minimal compared to the ethanolexpansion.

For example, assuming we have a 2.25 m diameter tank this will have aninitial circumference of 7.0685 m at an initial temperature. Thecoefficient of linear expansion of grade 316 stainless steel is 15.9μm/m/° C. If we assume a 20° C. rise from the initial temperature thenew circumference will be 7.0685+((15.9×7.0685×20)/1000,000)=7.0707 m.The cross sectional area at the initial condition=3.9761 Sqm and thecross sectional area at ±20° C. condition=3.9785 Sqm. The difference is0.0024 SqM. As the tank level for any capacity is directly related tocross sectional area, the error would be 0.0024/7.0685×15,000=5.09 L.This is for a 20° C. change so for a 1° C. temperature change this willbe 0.25 L or 0.0017%. While the tank is expanding, the ethanol will alsobe expanding. The coefficient of cubical expansion of ethanol is0.00109/° C. or 0.109%. The effective expansion is therefore0.109−0.0017=0.102%/° C.

A similar calculation can be carried out for the gasoline, making theassumption that the internals of the positive displacement meter do notchange significantly with temperature compared to the change of thegasoline volume.

It is helpful, therefore, to measure the temperature of the ethanol andthe gasoline in the tanks and correct the volume to a 20° C. reference.

Inaccuracy Due to Denaturing of the Ethanol

As mentioned above, ethanol in the ethanol storage tank 3 will often bedenatured by adding 1% volume of gasoline to the tank. Taking intoaccount the accuracy of the tank gauging and the accuracy of thegasoline addition it should be possible to achieve a 1.00%±0.004%Gasoline addition (between 0.996 and 1.004%).

As this gasoline will be contained in only 4.6 to 7.6% of the finalblend the error contribution will be small but the gasoline content ofthe ethanol must be taken into account in the final blending if thedesired mix is to be reached.

For example, if the user wishes to mix 100 volumes of gasoline and 4.6volumes of ethanol they will need to add slightly more ethanol as theethanol already contains 1% gasoline. So the user must add4.6/0.99×100=4.64646 volumes of denatured ethanol. This 4.64646 volumeshas 4.64646×0.01=0.04646 volumes of gasoline so we must add100−0.04646=99.9535 volumes of gasoline to it to achieve the 100:4.6ratio. If we correct the gasoline value back to 100% we need to add4.64862 (4.65) volumes of denatured ethanol to 100 volumes of gasolineto get a 100:4.6 ratio. A similar calculation would need to be appliedachieve a 7.6% addition, or any other desired amount.

Furthermore, where the ethanol is denatured by adding gasoline beforeuse, accurate tank gauging is required. The accuracy of this gaugingwill be affected by the accuracy of the tank dimensions. Typically, inembodiments of this invention, it is assumed that the tank is perfectlyround and that therefore a level measurement on the straight part of thetank will be directly related to the tank contents. However, the ethanolblending system 1 can be calibrated more accurately by filling theethanol storage tank 3 with water, and measuring the level reached forknown volumes of fluid.

Inaccuracy Due to Thermal Expansion in the Ethanol Injection Skid

Gasoline and ethanol expand/contract at slightly different rates withtemperature, ethanol at 0.109%/° C., gasoline at 0.100%/° C. The pistonsand cylinders will also expand and contract with temperature. As withthe tank above, the effect of expansion on the cylinders and pistons isminimal, of the order of 0.0017%/° C. in terms of volume.

As long as the gasoline and ethanol cylinders are kept at the sametemperature this effect is compensated for. Also as long as thetemperatures of the cylinders are exactly the same the mixing ratiowould be maintained over quite a wide temperature range (>±10° C.).However, if the temperatures differ the expansion effects are moresevere. The ratio would be theoretically maintained within a ±3° C.temperature difference. So the target for the overall temperaturecontrol in the ethanol blending system 1 is to ensure that thedifferential temperatures do not exceed 1 to 2° C.

Inaccuracy Due to Fluid Compression

Both alcohol and gasoline are compressible to some extent. Once againthe rates differ, especially for gasoline which does not have a constantcomposition. Ethanol is compressible to the extent of about 0.01%/bar,while gasoline is less compressible. Also the degree of compressibilitydecreases with pressure, which makes the effects of compressibility hardto predict.

Ideally the mixing should be carried out at atmospheric pressure, or aslow a pressure as possible to eliminate the potential of compressibilityerrors. Typically, a system according to the invention is limited by thepressure requirements of the installed analysers to a minimum pressureof 7 bars.

To minimise compressibility errors the pressure of the ethanol ismaintained close to the pressure of the incoming gasoline by means of anelectrically controlled pressure regulator.

Testing

For factory acceptance testing the function and accuracy of the ethanolblending system 1 is determined gravimetrically using water, asdescribed above. A gravimetric procedure is also used to calibrate thecheck cylinder volumes.

For site acceptance testing and routine checking the check cylinders areused, as also described above. Alternatively, gas chromatography can beused for testing accuracy. Lastly, a photometric method using a suitablespectrometer and Sudan Blue Dye can also be used.

The ethanol used would not be 100% pure so the level of impurities inthe ethanol and the added gasoline would have to be taken into account.Therefore any check would also involve determining the ethanol contentof the tank as well as the ethanol content of the blend.

1. A method for providing a fuel meeting a predetermined specificationof properties, the method comprising: taking a sample of the fuel;mixing an additive into the sample in metered proportions; testing thesample to determine that the combination of the fuel and the additivemeets the predetermined specification of properties; storing theremaining fuel without the additive for subsequent mixing with theadditive into the remainder of the fuel in the same metered proportion.2. The method as claimed in claim 1, further comprising the step of:adjusting the metered proportions of the fuel and the additive wheretesting reveals that the sample contains an incorrect amount ofadditive.
 3. The method as claimed in claim 1, wherein the fuel and theadditive are fluids at the time of mixing.
 4. The method as claimed inclaim 3, wherein at least one of the fuel and the additive are liquid atthe time of mixing.
 5. The method as claimed in claim 1, wherein atleast one of the fuel and the additive is a hydrocarbon.
 6. The methodas claimed in claim 5, wherein the hydrocarbon has been distilled fromcrude oil.
 7. The method as claimed in claim 5, wherein one of the fueland the additive comprises gasoline.
 8. The method as claimed in claims5, wherein one of the fuel and the additive comprises alcohol.
 9. Themethod as claimed in claim 8, wherein the alcohol comprises ethanol. 10.The method as claimed in claim 1, wherein one of the fuel and theadditive comprises a bio-fuel.
 11. An apparatus adapted for carrying outthe method of claim 1, the apparatus comprising; a blending system forincorporating the additive into the fuel; and a sample line for takingoff a sample of the mixed additive and fuel for testing.
 12. Theapparatus as claimed in claim 11, wherein the blending system comprisesa skid.
 13. The apparatus as claimed in claim 11, wherein the blendingsystem comprises a plurality of cylinders, and wherein each cylindercontains a piston, and each cylinder comprises at least one inlet,through which fuel or additive is supplied to the cylinder.
 14. Theapparatus as claimed in claim 13, wherein each cylinder comprises afirst inlet and a second inlet, one at each end of the cylinder, and avalve which in use alternately directs the fuel or additive to the firstinlet or the second inlet.
 15. The apparatus as claimed in claim 14,wherein, in use, the piston of each cylinder is driven solely by thepressure of fluid entering the cylinder.
 16. The apparatus as claimed inclaim 13, wherein the blending system comprises a primary cylinder andat least one secondary cylinder, wherein the pistons in the secondarycylinders are arranged to operate in synchrony with the piston in theprimary cylinder.
 17. The apparatus as claimed in claim 16, wherein eachcylinder comprises a first inlet and a second inlet, one at each end ofthe cylinder, and a valve which in use alternately directs the fuel oradditive to the first inlet or the second inlet, wherein the primarycylinder comprises at least one proximity switch, arranged to operatethe valve in the primary cylinder when the piston approaches the end ofthe primary cylinder.
 18. The apparatus as claimed in claim 17, whereinthe proximity switch in the primary cylinder is also arranged to operatethe valves in the secondary cylinders.
 19. The apparatus as claimed inclaim 11, wherein the apparatus comprises a heat exchanger, and the fueland the additive are put through opposite sides of the heat exchangerprior to mixing.
 20. The apparatus as claimed in claim 11, wherein thecylinders are mounted inside a temperature controlled enclosure.